A few days ago, I posted a small JavaScript snippet.
Retina, Please! is that JavaScript snippet on steroids. It is a combination of JavaScript and PHP to allow your Retina users to only download Retina images, while non-Retina users only download the standard-resolution version.
Check it out: https://github.com/keichan34/KKRetinaPlease
WordPress Theme Writers: just require it in your functions.php. I may package it into a real plugin later on.
I’ve tested it with CakePHP as well.